Background Chiropractic spinal manipulation treatment (SMT) is common for back pain and has been reported to increase the risk for lumbar disc herniation (LDH), but there is no high quality evidence about this. In the absence of good evidence, clinicians can have knowledge and beliefs about the risk. Our purpose was to determine clinicians’ beliefs regarding the risk for acute LDH associated with chiropractic SMT.

Methods Using a belief elicitation design, 47 clinicians (16 chiropractors, 15 family physicians and 16 spine surgeons) that treat patients with back pain from primary and tertiary care practices were interviewed. Participants’ elicited incidence estimates of acute LDH among a hypothetical group of patients with acute low back pain treated with and without chiropractic SMT, were used to derive the probability distribution for the relative risk (RR) for acute LDH associated with chiropractic SMT.

Conclusions Clinicians’ beliefs about the risk for acute lumbar disc herniation (LDH) associated with chiropractic SMT varied systematically across professions, in spite of a lack of scientific evidence to inform these beliefs. These probability distributions can serve as prior probabilities in future Bayesian analyses of this relationship.

Spinal manipulation is commonly used for back pain and considered to be safe, although concern has been raised about its potential link with lumbar disc herniation (LDH) [1, 2]. Several systematic reviews have found that spinal manipulation can benefit low back pain, with little evidence of serious harm [3, 4]. In addition, randomized clinical trial evidence supports the use of spinal manipulation for the treatment of LDH with radiculopathy [5–7]. On the other hand, some believe, on the basis of case reports and small case series, that spinal manipulation is contraindicated for the treatment of disc herniation and could cause it [8–14]. Therefore, a disconnect seems to exist between the scientific evidence and some clinical beliefs in this area.

Understanding of the risk for serious adverse events related to spinal manipulation faces several challenges. First, there has been insufficient study in this area. No valid estimate of the risk for acute disc herniation following chiropractic spinal manipulation treatment is available in the scientific literature. Second, the valid study of uncommon events, such as acute LDH with radiculopathy following spinal manipulation treatment, is limited by the rarity of this outcome. Well-designed prospective studies are difficult to undertake due to the challenge of recruiting and following the numbers of patients needed to ensure that sufficient events are observed. Third, methodological pitfalls exist. For instance, case–control designs based on retrospective measurement of exposure to hypothesized iatrogenic harm are highly susceptible to information biases including recall and observer bias. Fourth, a necessary prerequisite for interest in scientific pursuit is a finding of community uncertainty [15] (a situation where not all within a community of “experts” agree on the potential risk of an intervention).

A Bayesian approach to health services research and evaluation
presents an opportunity to advance knowledge despite
the above challenges [16, 17]. Bayesian methods start with
existing “prior” beliefs, formally quantified as probability
distributions, and update these using new data to arrive
at “posterior” beliefs, which may be used as the basis for
inferential decisions [18]. Quantifying currently held beliefs
can determine the magnitude of a potential risk expected by
experts and describe the presence of uncertainty, discrepancies
or clinical equipoise about an exposure–outcome association.
Experts in a field can have knowledge of the risk of
using a treatment through years of clinical experience. When
quantified, the knowledge gained from their clinical experience
can be included in Bayesian models estimating risk and
may help to bridge the gap between beliefs and evidence. A
particular strength of this approach is that probability distributions
obtained through the elicitation of beliefs can be
used to augment scarce data [16, 17, 19, 20], and be formally
incorporated into Bayesian risk analyses. Beliefs about risk
or harm influence clinicians’ practice and may also influence
their receptiveness to new research findings. In the absence
of risk estimates based on data, experts may be the best currently
available source of evidence. Although infrequently
used in epidemiologic research, a Bayesian approach may
have important utility in the study of treatment-related rare
serious adverse events, particularly in the absence of definitive
scientific evidence [16, 17, 21, 22].

Our objective was to describe and quantify clinicians’
beliefs about the association between chiropractic spinal
manipulation and the development of acute LDH. We elicited
beliefs from three groups of clinicians that treat patients
with back pain and disc herniation: chiropractors, family
physicians and spine surgeons. We hypothesized that beliefs
about the risk following chiropractic care would vary by
clinician group, with chiropractors believing spinal manipulation
to be less risky than spine surgeons.

Methods

Study participants

Since belief elicitation is best conducted through a face-toface
interaction between participant and investigator [19,
23], we considered eligible only those participants available
for an interview within a 2-h drive of the City of Toronto.
Chiropractors were sampled from active registered members
with the College of Chiropractors of Ontario within
the Greater Toronto Area. Family physicians were sampled
from primary care active registered members with the College
of Physicians and Surgeons of Ontario who refer to a
spine surgery service at a tertiary care hospital in Toronto.
Spine surgeons invited to participate were all eligible active
members of the Ontario Division of the Canadian Spine
Society. We also imposed a requirement (self-reported) that
the clinician’s practice involve caring for patients with low
back pain and LDH.

There is no consensus on the preferred sampling method
or sample size for a belief elicitation study [19, 23, 24].
A systematic review of belief elicitation methods found a
median sample size in elicitation studies of 11 participants,
and recommended the use of purposive sampling to capture
a range of opinions of the target population [23]. We hypothesized
that beliefs might vary on the basis of clinician group,
years of clinical experience or gender, so in our intended
purposive sample of 15 members of each clinician group,
we attempted to balance these characteristics.

Study design

We conducted a belief elicitation study using a computer
adaptation of a validated approach to belief elicitation that
has previously been used with various groups of clinicians
[24–26]. A face-to-face interactive interview with each participant
was conducted by one interviewer (CAH), using a
standardized questionnaire, script and elicitation software
(Online Resource 1 and 2). The interview, script and software
were pilot tested with a group of ten clinicians (six
chiropractors, two family physicians and two spine surgeons)
for face and content validity, clarity and time feasibility
(≤15-min duration), with good results: 9/10 found the questions
clear; 9/10 found the response option easy to use; the
median time to completion was 15 min. We obtained ethics
approval for our study from the research ethics board of the
University Health Network (REB #11-0240-AE).

Belief elicitation interview

The interview incorporated best practices for belief elicitation
[19, 23, 24], and began with a short introduction, followed
by informed written consent. The participant was
then guided through a worked example (Online Resource
1, Example Scenario) of a belief elicitation to introduce the
elicitation process and computer software. Following this,
beliefs were elicited for two clinical outcome scenarios:
acute LDH and acute severe LDH that is surgically managed
(we report on the first of these scenarios here; for information
on the second scenario, see doctoral thesis [27]). Acute
LDH was defined as acute onset of lumbar radiculopathy
due to LDH. For each scenario, a participant was asked to
consider the following at-risk population: a hypothetical
average group of 1000 newly diagnosed acute low back pain
patients without current LDH. We used a denominator of
1000 because it is an observable and conceivable sample of
patients. This facilitates the probabilistic mental manipulations
that have to be made during the elicitation interview,
increases the reliability of the elicitation, and reduces some
of the common elicitation biases (e.g., tendency to think in
terms of percentages) [19, 23, 24].

Each participant was asked to estimate the 2-month
cumulative incidence of acute LDH among an average group
of 1000 newly diagnosed acute low back pain patients without
current LDH that are treated with and without chiropractic
care. Chiropractic care was defined as a course of
spinal manipulation to the lumbar spine by a chiropractor.
Participants were also asked to indicate their uncertainty
around their estimate among chiropractic-treated patients by
expressing the lower and upper bounds of what they believed
to be a plausible range for the incidence. Using the belief
elicitation software (detailed below), participants were then
asked to indicate the distribution of the weight of their belief
along this plausible range.

We also asked participants to categorize their belief
regarding the overall effect of chiropractic care, compared
to primary medical care (defined as a course of NSAIDs/
painkillers prescribed by a family physician), on the risk
of developing acute LDH, using the response options:
“decreases the risk,” “no effect on the risk,” or “increases
the risk.”

Finally, we recorded other demographic and clinical
characteristics of the participants (e.g., gender, age group,
clinical background, number of years in practice treating low
back pain patients, number of new low back pain patients
seen per year) to gather some descriptive information.

Belief elicitation software

We used an open source computer protocol (modified
SHELF [28] application in R [29]) that allowed participants
to graphically specify a point estimate of incidence and a
plausible range of incidence values that they believed likely
for acute LDH among patients treated with chiropractic care
(Online Resource 3 provides screenshots of the computer
interface and details of the modifications we made to the
base SHELF software). They indicated their weight of belief
for the incidence values along their specified range by allocating
20 virtual “chips”, each representing 5% probability,
thereby creating a probability distribution representing 100%
probability, from which the software estimated a smooth
curve of best fit. This curve provided participants a real-time
graphical representation of their probability distributions.
Participants were asked to review the placement of chips and
the smooth curve to ensure that these accurately reflected
their true belief prior to proceeding in the interview.

Statistical analysis

Medians and proportions were used to summarize characteristics
of the respondents. On each participant, the distribution
of the absolute risk of acute LDH with chiropractic
spinal manipulation was converted to the distribution for the
relative risk (RR). To estimate the RR, values of the absolute
risk of acute LDH among patients treated with chiropractic
spinal manipulation (the values on the x-axis of the bottom
figure in Online Resource 3) were divided by the point estimate
of incidence of acute LDH among untreated patients.

We aggregated the smooth best-fitting curve from each
participant to generate an overall summary of belief about
the RR. We also aggregated curves within each of the clinician
groups. Finally, optimistic and pessimistic probability
distributions were estimated by aggregating respondents
within the lowest and highest quartiles of mean RR estimates,
respectively. For each summary curve, the probability
placed on a RR greater than one (RR > 1) was calculated as
the area under the curve for RR values greater than 1.

Prespecified secondary analyses were undertaken to
describe and quantify beliefs by gender, clinician age
(≤50 years and ≥51 years), and years of clinical experience
(<12, 12–24 and ≥25). We used R, version 3.1.2 [29] to
carry out our statistical analysis.

Role of the funding source

The sponsors of the study had no role in the study design,
data collection, data analysis, data interpretation, the writing
of the report, or in the decision to submit the report for
publication.

Results

Study participants

Of 165 eligible potential participants invited, 87 did not
respond and 31 declined to participate (reason not given).
Overall participation was 28% (47/165), and ranged from
70% among spine surgeons (16/23), to 37% among chiropractors
(16/43), and 15% among family physicians (15/99).

Table 1 describes characteristics of the study participants.
Spine surgeons were all men and on average older than the
other groups. Most participants (94%) reported some formal
post-secondary statistical training and had many years
of experience treating low back pain patients (median 18;
IQR 10–26 years). Chiropractors had slightly less experience
(median 13 years) and spine surgeons more (median
23 years).

Clinicians’ beliefs about the baseline risk for acute LDH

The median elicited 2-month incidence of acute LDH among
patients with acute low back pain not treated with chiropractic
care across all participants was 5.0% (IQR 2.3–10.0%).
All three clinician groups expressed very similar beliefs
about the baseline risk for acute LDH.

Clinicians’ beliefs about the risk for acute LDH associated with chiropractic care

The median RR for acute LDH across all clinicians was
0.97 (IQR 0.57–1.20), with a probability for a RR > 1 of
46.7% (Figure 1a; Table 2). Beliefs varied by clinician group
(Fig. 1b), with chiropractors reporting the most optimistic
belief (median RR 0.56, IQR 0.39–1.03; probability for
a RR > 1 26.7%). Family physicians held a more neutral
belief, with a median RR of 0.97 (IQR 0.64–1.21) and
probability for a RR > 1 of 47.1% (Table 2). Bimodal
probability distributions were observed for both chiropractors
and family physicians (Fig. 1b), reflecting the
presence of two subgroups for each clinician type: one
believing there is a protective effect of spinal manipulation
on the risk for acute LDH with radiculopathy, and
another believing there is not only little effect on average
but also some chance of harm. Spine surgeons reported a
slightly more pessimistic belief, with a median RR of 1.07
(IQR 0.95–1.29) and probability for a RR > 1 of 64.8%.
The unimodal distribution for the spine surgeons showed
them to be a more homogeneous group, which on average,
felt there was close to no benefit, but with more chance of
harm than benefit.

Optimistic clinicians—those in the lowest quartile of RR
estimates—included three family physicians and nine chiropractors.
These clinicians expressed a belief that chiropractic
spinal manipulation reduces the incidence of acute LDH
by about 60% (median RR 0.42; IQR 0.29–0.53) (Figure 1c; Table 2). Pessimistic clinicians—those in the highest quartile of RR estimates—included six spine surgeons, three
family physicians and three chiropractors, who believed that
chiropractic care increases the incidence of acute LDH by
about 30% (median RR 1.29; IQR 1.11–1.59). Compared
to optimists, pessimists were more commonly men, older
and reported a greater number of new low back patients per
year (Table 3). More pessimists than optimists also reported
having seen a patient whose disc herniation was attributed to
chiropractic spinal manipulation (58 versus 8%).

Beliefs by gender, age groups and years of clinical experience

Table 2 shows that some differences in belief about the risk
for acute LDH associated with chiropractic care were found
between women (n = 11) and men (n = 35) clinicians, with
women reporting a more optimistic belief (median RR 0.77;
IQR 0.41–1.10; 34% probability for a RR > 1) and men,
a more neutral belief (median RR 1.00; IQR 0.68–1.23;
51% probability for a RR > 1). There were no differences
in beliefs according to age group or years of clinical experience
treating patients with low back pain (Table 2).

Table 3.
Characteristics of clinicians
with optimistic and pessimistic beliefs
about the risk for acute LDH associated
with chiropractic spinal manipulation treatment

Discussion

This belief elicitation study quantified, described and provided
important insights into beliefs about the risk for acute
LDH associated with chiropractic spinal manipulation held
by clinicians with experience treating patients with back
pain and disc herniation. Clinician beliefs about the incidence
of acute LDH with and without chiropractic spinal
manipulation treatment varied, with interesting and important
differences observed across professions and specializations.
On average, clinicians believed that the incidence of
acute LDH is about the same with and without chiropractic
spinal manipulation treatment; in other words, clinicians
believed that exposure to chiropractic care has no major
effect on the risk for acute LDH. However, it is striking to
note the stark difference in beliefs across professions and
between optimistic and pessimistic clinicians (Fig. 1). There
is a divergence of opinion within these back pain clinicians,
indicating the presence of community uncertainty.

Our study advances knowledge of the potential link
between chiropractic spinal manipulation and acute LDH
in several important ways. First, our study is the first to
examine and provide estimates of clinician beliefs regarding
this potential exposure–outcome association. There are no
published estimates of the association between chiropractic
care and the incidence of acute LDH with which to compare
our findings. In the absence of estimates of risk based on
data, experts may be the best currently available source of
evidence. By quantifying currently held clinician beliefs,
our results shed light on the magnitude of a potential risk
expected by experts and describe the community uncertainty
surrounding this relationship. Experts in a field can have
knowledge of the risk of using a treatment through years
of clinical experience. Beliefs about risk or harm influence
clinicians’ practice and will also influence their receptiveness
to new research findings. Second, the probability distributions
elicited in this study may be relevant and useful
to researchers choosing to use a Bayesian approach to investigate
this relationship, whereby elicited beliefs in the form
of prior probability distributions can be used to augment
limited data [19–21, 30, 31]. Finally, our results act as a
benchmark against which future quantitative studies could
be assessed using Bayesian methods. For example, if a future
study found harm associated with spinal manipulation,
one could evaluate whether the evidence should be strong
enough to convince chiropractors as a group, or, in the whole
sample, persons with the most strongly held beliefs about
benefit. Likewise, if a future study found benefit associated
with SMT, one could assess if the new evidence should be
compelling enough to persuade those with the most strongly
held beliefs about harm to update their beliefs. The Bayesian
approach appears particularly reasonable and appropriate in
the context of investigating contentious potential treatmentrelated
rare serious adverse events in the absence of definitive
scientific data and evidence [16, 17].

Our study has limitations that need to be considered.
These mainly relate to characteristics of the study respondents.
One of the ongoing debates in the belief elicitation
literature is the definition of an “expert” when eliciting
expert opinion [19, 23, 24]. As there is no formal definition
for back pain clinician expert, we defined an “expert”
as a chiropractor, family physician or spine surgeon with a
self-reported active practice treating patients with low back
pain. The large number of years treating patients with low
back pain and large number of new patients with low back
pain seen per year (Table 1) supports the notion that the participants
were experienced clinicians. However, the overall
response rate to our survey was low. If reasons for study
participation were associated with beliefs regarding chiropractic
care safety, then our findings could be subject to nonresponse
bias. We hypothesize that respondents may have
had more extreme beliefs than nonrespondents (i.e., those
with strong opinions might have been more likely to participate).
If this was the case, our findings may overestimate
differences between clinician groups and between optimists
and pessimists. However, if the propensity to respond was
similar in those with extreme positive and extreme negative
views, then our average relative risk may be representative
of that in the target population. We would have preferred
a more balanced study sample with respect to gender and
age distribution. However, this was challenging, as Ontario
spine surgeons are predominantly male and older compared
with the more balanced family physician population and the
younger population of chiropractors in the Greater Toronto
Area (Table 1). We acknowledge that our outcome measurement
did not determine if respondents believed that LDH
specifically occurred at the lumbar spinal segment treated
with SMT or at a lumbar segment distant from the lumbar
spine area treated. Finally, this is an exploratory study
involving a relatively small number of clinicians that may
not be representative of the clinician source populations.
However, our main objective was to describe the beliefs of
our study sample and generate hypotheses, rather than to
make strong generalizable inferences about beliefs in these
healthcare professions.

Our study is part of a growing body of literature on the
successful quantification of clinicians’ beliefs about the
safety or effectiveness of an intervention or treatment exposure.
For example, this method has been used to elicit beliefs
regarding: treatment efficacy in pulmonary arterial hypertension
[25] and pediatric intestinal failure-associated liver disease
[26]; and risk factors for falls in community-dwelling
older people [32]. The belief elicitation methodology used
in our work can be generalized to the study of many uncommon
diseases and conditions [19], and could have potential
web-based research applications [33].

A strength of our elicitation procedure is that participants
were asked to estimate the incidence of acute LDH with and
without chiropractic spinal manipulation treatment, rather
than to directly provide their belief about the relative risk.
The probability distribution for the relative risk was derived
by first obtaining the estimate of risk without chiropractic
care and then, with this in mind, obtaining the distribution of
incidence with chiropractic care. The elicited incidence with
chiropractic care was conditional on the incidence without
chiropractic care, which acted as a baseline value. We took
this approach because evidence shows that it is much more
difficult for respondents to directly specify relative risk distributions
as relative risks are not directly observable [19].
A clinician can reflect on knowledge and experience to estimate
a frequency representation of the probability for acute
LDH, however, it is a much more challenging task to abstract
that information and elicit a valid distribution for the ratio
of probabilities (i.e., the RR).

Interesting questions remain. What are the reasons for the
differences in clinician beliefs? Do these clinicians interpret
the same evidence, as limited as it is, differently? Do
they see different types of patients, so that their beliefs actually
reflect their personal clinical experience? Does their
understanding of the pathogenesis of LDH and its associated
clinical presentation affect the manner in which different
health professionals are likely to view LDH? Does
spine surgeons’ experience with the more severe end of the
clinical spectrum raise their index of suspicion for viewing
spinal manipulation as potentially harmful? Do they
have built-in biases based on their professions and specializations?
Would similar beliefs be observed in healthcare
settings where the education of these clinician groups is
more integrated (e.g., Denmark [34], Switzerland [35]), or
where family physicians and spine surgeons deliver spinal
manipulation? In Germany, for example, general practitioners
and ambulatory orthopaedic surgeons are often specially
trained in spinal manipulation and offer it to their patients
[36]. This highlights the need for future well-designed studies
to examine determinants of beliefs, confirm clinicians’
beliefs about the safety of chiropractic spinal manipulation
in other settings, and investigate the association between
chiropractic care and acute LDH in the population. More
research is warranted to better understand the risk–benefit
profiles of manual therapeutic interventions for spinal pain
and dysfunction [37, 38].

Conclusion

Our study is the first to describe and evaluate clinician
beliefs about the association between chiropractic spinal
manipulation and the development of acute LDH. We
found that beliefs were diverse, with important interprofessional
differences. Our probability distributions quantify
and describe these differing beliefs and can be used as prior
probabilities in Bayesian analyses of this relationship.

Acknowledgements

We thank Drs. Sindhu Johnson and Ivan Diamond
for their assistance with the development of the belief elicitation
interview and questionnaire. We acknowledge Drs. Jeremy Oakley and
Anthony O’Hagan for their development and open source sharing of
the SHELF elicitation framework. We thank all the clinician experts
who participated in the belief elicitation interviews.

Author contributions

CAH conceived and designed the study,
acquired, cleaned and analysed the data, interpreted the results, and
drafted and revised the paper. GT conceived and designed the study,
analysed the data, interpreted the results, and revised the paper. PC,
YRR, ARJ and JDC designed the study, interpreted the results, and
revised the paper. CAH acts as guarantor.

Funding sources

Canadian Institutes of Health Research [Grant ID:
200902KPD-205299-111612], and Canadian Chiropractic Research
Foundation.

Conflict of interest

All authors have completed the ICMJE uniform
disclosure form at http://www.icmje.org/coi_disclosure.pdf (available
from the corresponding author) and declare: CAH was funded by Fellowship
Awards in the Area of Knowledge Translation from Canadian
Institutes of Health Research [Grant ID: 200902KPD-205299-111612]
and Canadian Chiropractic Research Foundation, PC is funded by a
Canada Research Chair in Disability Prevention and Rehabilitation
at the University of Ontario Institute of Technology; PC consults for
European Spine Society, National Judicial Institute of Canada, Canadian
Memorial Chiropractic College, Canadian Chiropractic Protective
Association, Society of Musculoskeletal Manual Practitioners of
Saskatchewan, Société des experts en évaluation médico-légale du
Québec, YRR consults for Medtronic, outside the submitted work; no
other relationships or activities that could appear to have influenced the
submitted work. The funders played no role in the design and conduct
of the study; collection, management, analysis, and interpretation of
the data; and preparation, review, or approval of the manuscript.